Retroreflective articles have the ability to return a substantial portion of incident light in the direction from which the light originated. This unique ability has lead to the widespread use of retroreflective articles on clothing worn by highway construction and maintenance workers and fire-fighters. The retroreflective articles displayed on their clothing typically are in the form of retroreflective stripes. The retroreflective stripes typically comprise a layer of microspheres such as glass beads. The retroreflective articles improve the wearers' safety by highlighting their presence.
Several basic types of microsphere-containing retroreflective materials are known. On the one hand, so-called embedded or encapsulated lens type sheetings are known in which the microspheres are covered by a transparent resin layer, i.e., they are fully buried and not exposed to air. The second type of reflective sheeting having microspheres is the so-called open-bead or open-lens material in which the microspheres are partially exposed to air, i.e., they are not completely buried in a binder layer. A third type of microsphere sheeting is similar to the second type, with the exception that a polymeric cover film is heat-sealed intermittently over the microsphere-bearing surface of the reflective sheet. The microspheres in the enclosed lens sheeting are exposed to air (beneath the polymeric cover film), but are not exposed to the elements such as rainfall and are not considered to be open-bead sheeting.
A particular disadvantage of the open-bead reflective sheeting is its reduced reflectivity under rainfall conditions. Moreover, the reflectivity of the sheeting often diminishes after several launderings.
JP 08-309929 discloses treating the exposed glass bead of an open-bead type reflective sheet with a combination of a fluorochemical compound and a silane coupling agent. As the fluorochemical compound, there is taught a perfluoroalkyl acrylic acid ester. Also, it is recommended to additionally use a melamine resin or an isocyanate crosslinking agent so as to further improve the durability of the treatment.
EP 1,262,802 provides a reflective sheet that comprises a reflective element and that comprises microspheres partially exposed at a major surface of the reflective sheet. The reflective sheet has further been treated with a fluorinated silane compound that has a fluorinated group and a silane group having one or more hydrolyzable groups.
While some of the known surface treatments may be capable of providing acceptable levels of initial repellent properties, a loss of repellency is often encountered due to repeated launderings. Further, it would be desirable to improve the reflective properties of the open-bead reflective sheet material under rainfall conditions.
Accordingly, it is desirable to provide a coating composition capable of providing a high durable water repellent coating on a retroreflective sheet. In particular, it is desirable to provide a durable coating wherein the initial reflective and repellent properties are substantially maintained, even under repeated launderings. It will typically also be desired that the treatment composition has good storage stability, minimal environmental disadvantages and can be conveniently manufactured at minimal costs.